The laser gyro, developed around thirty years ago, is widely commercialized and used these days. Its operational principle is based on the Sagnac effect, which leads to a frequency difference ΔF between the two optical transmission modes propagating in opposite directions, called counterpropagating, of a bidirectional ring laser cavity when it is stimulated by a rotational movement. Conventionally, the frequency difference ΔF is equal to:ΔF=4A ω/λL where L and A are the optical length and the area of the cavity respectively; λ is the wavelength of the laser transmission without the Sagnac effect; ω is the speed of angular rotation of the laser gyro.
The measurement of ΔF obtained by spectral analysis of the beating of the two transmitted beams enables the value of ω to be known with very great precision.
Electronically counting the interference beats that pass during a change in angular position also enables the relative value of the angular position to be known with very great precision.
The production of laser gyros presents certain technical difficulties. A first difficulty is linked with the quality of the beating between the two beams, which determines the proper operation of the laser gyro. This is because good stability and relative equality of the intensities transmitted in the two directions is necessary to obtain correct beating. For in the case of solid-state lasers this stability and this equality are not ensured due to the phenomenon of intermodal competition, which causes one of the two counterpropagating modes to tend to monopolize the available gain to the detriment of the other mode. The problem of instability of the bidirectional transmission for a solid-state ring laser may be solved, for example, by fitting a negative feedback loop intended to control the difference between the intensities of the two counterpropagating modes around a fixed value. This loop acts on the laser either by making its losses dependent on the direction of propagation, for example by means of an assembly comprising an element inducing a reciprocal rotation, an element inducing a nonreciprocal rotation and a polarizing element (patent application no. 03/03645), or by making its gain dependent on the direction of propagation, for example by means of an assembly comprising an element inducing a reciprocal rotation, an element inducing a nonreciprocal rotation and a polarized emission crystal (patent application no. 03/14598). Once controlled, the laser emits two counterpropagating beams, the intensities of which are stable and may be used as a laser gyro.
A second technical difficulty is linked with the domain of low rotation speeds, the laser gyro only operating correctly beyond a certain rotation speed. At low rotation speeds the Sagnac beat signal disappears because of a coupling between the two counterpropagating modes due to backscattering of the light caused by the reflectors and the various optical elements possibly present in the cavity. The domain of low rotation speeds for which this phenomenon is produced is commonly called the “dead zone”. This problem is not intrinsic to the solid state. It is also encountered in the gas laser gyro field. The most commonly adopted solution for this latter type of laser gyro consists in mechanically activating the device by imposing on it a forced periodic movement that artificially puts it outside the dead zone as often as possible. Another solution consists in introducing a constant phase shift between the two optical paths, which results in a frequency bias between the two counterpropagating modes. Hence, the operational field of the laser gyro is artificially moved outside the dead zone. However, this latter solution has the significant drawback that the value of the bias introduced must be perfectly stable over time.
A third difficulty is linked, in the context of solid-state lasers, with the fact that the counterpropagating waves interfere in the amplifying medium, creating a population inversion grating. In fact, if the amplifying medium is a solid Nd:YAG crystal, it is possible to show that in such a medium the population inversion gratings caused by stimulated emission in the gain medium has the effect of destabilizing the bidirectional emission. In addition, when the laser gyro is rotating, these gratings become mobile and through the Doppler effect cause a frequency shift between the two counterpropagating waves circulating in the laser cavity, which increases the nonlinearity of the frequency response of the laser gyro.